The present invention relates to a process for producing lower alkyl fatty esters from fatty acids in fixed-bed reactor(s) charged with a weakly acidic solid catalyst by countercurrent or pseudo-countercurrent operation.
The lower alkyl fatty esters have been produced from of old as the starting material for producing higher alcohols, and methyl fatty esters are generally obtained by reaction of triglycerides with methanol or by reaction of fatty acids with methanol. In the reaction of triglycerides with methanol, glycerin is formed as a byproduct, but the formed methyl fatty esters and glycerin are easily separated as different phases, and thus the reaction easily proceeds. On the other hand, the reaction of fatty acids with methanol is an equilibrated reaction so that unless water formed as a byproduct is efficiently removed, the apparent rate of the reaction is lowered or the equilibrium of the reaction is established at a higher concentration of remaining fatty acids, and thus the amount of the remaining fatty acids cannot be sufficiently lowered.
To solve these problems, JP-A 1-283251 discloses a method of using a multi-stage-plate reaction column and reducing the pressure in the top of the reaction column. In this method, removal of water is improved by reduced pressure, but the catalyst used in esterification is a uniform catalyst, so removal of the catalyst from the desired products i.e. methyl fatty esters is necessary.
On the other hand, JP-B 53-6161 discloses a process for producing lower alkyl fatty esters by means of a fixed bed. It is described that in this process, a mixed solution of fatty acid and alcohol is brought into contact with a catalyst layer and the alcohol is gasified, but in this process, the operation of only co-current of fatty acid and alcohol is conducted, and water formed as a byproduct is evaporated into the gas phase but partially remains from the relationship of gas-liquid equilibrium. As the reaction proceeds, the content of water in the gas phase particularly in the vicinity of an outlet of the reaction column is increased so that from the relationship of gas-liquid equilibrium, the content of water in the liquid phase is also increased, thus making it difficult to lower the amount of the remaining fatty acid because of the equilibrated reaction. For lowering the content of water in the liquid phase, a method of using a large amount of alcohol can be anticipated but is not economical.
JP-A 7-224002(JP-B2 2707063) discloses a fixed-bed system for esterification wherein a strong acid catalyst and ion-exchange resin are used as the catalyst.
The object of the present invention is to provide a process for producing lower alkyl fatty esters from fatty acids and lower alcohols, wherein lower alkyl fatty esters are produced in higher yield with a reduction in the amount of the remaining fatty acids.
The invention provides a process for producing a lower alkyl fatty ester, which comprises feeding a fatty acid and a lower alcohol in a fixed-bed reactor charged with a weakly acidic solid catalyst and reacting them with each other by bringing the fatty acid into contact with gas of the lower alcohol in countercurrent operation in the bed.
The invention also provides a process for producing a lower alkyl fatty ester, which comprises feeding a fatty acid and a lower alcohol in at least two fixed-bed reactors charged with a weakly acidic solid catalyst and reacting them with each other by bringing the fatty acid into contact with gas of the lower alcohol in co-current operation in one of the reactors and then in countercurrent operation in the other reactor. It is here preferable that the gaseous lower alcohol is first fed to the countercurrent fixed-bed reactor and then gaseous lower alcohol discharged from the outlet of the reactor is fed to the co-current fixed-bed reactor.
The invention then provides a process for producing a lower alkyl fatty ester, which comprises feeding a fatty acid and a lower alcohol in multi-staged fixed-bed reactors each charged with a weakly acidic solid catalyst and reacting them with each other by feeding the fatty acid to a reactor at an upstream stage and sending it to a stage at the downstream side, feeding the gaseous lower alcohol to a reactor at a downstream stage to carry out downward co-current operation and at the same time returning the gaseous lower alcohol discharged from the outlet of the reactor to a stage at the upstream side to repeatedly conducting a pseudo-countercurrent operation in the fixed bed of each reactor.
The fatty acids used in this invention include, but are not limited to, saturated or unsaturated fatty acids obtained by hydrolysis of natural vegetable and animal fats and oils. The vegetable fats and oils include e.g. coconut oil, palm oil, palm kernel oil, soybean oil etc. The animal fats and oils include e.g. tallow, lard, fishoil etc. Further, organic acids such as dicarboxylic acids and tricarboxylic acids can be mentioned. These are used preferably in a liquid form.
The lower alcohols used in the present invention are preferably C1-5 lower alcohols. Specifically, monoalkanols such as methanol, ethanol and propanol can be mentioned, and methanol is industrially preferable because of low cost and easy recovery.
It is preferable the weakly acidic solid catalyst has a strong acid point of not higher than 0.2 mmol/g-Cat and a weak acid point of not less than 0.3 mol/g-Cat, each acid point being defined as follows:
Weak acid point: the point at which desorption of NH3 occurs in the range of 100 to 250xc2x0 C. in TPD (ammonia adsorption-desorption process);
Strong acid point: the point at which desorption of NH3 occurs at a temperature higher than 250xc2x0 C. in TPD.
It is further preferable that the weakly acidic solid catalyst is a molded article of a weakly acidic solid catalyst having the structure (A), the structure (B) and the metal atom (C) as follows:
Structure (A): a structure of an inorganic phosphoric acid wherein the hydrogen atom is removed from at least one OH group thereof,
Structure (B): a structure of an organic phosphoric acid represented by the formula (1) or (2), wherein the hydrogen atom is removed from at least one OH group thereof: 
wherein xe2x80x94R1 and xe2x80x94R2 is independently selected from the group consisting of xe2x80x94R, xe2x80x94OR, xe2x80x94OH and xe2x80x94H and at least one of xe2x80x94R1 and xe2x80x94R2 is xe2x80x94R or xe2x80x94OR, xe2x80x94R being a C1-22 organic group, and
Metal atom (C): at least one metal atom selected from the group consisting of aluminum, gallium, and iron.
It is much preferable that the weakly acidic solid catalyst is a molded article of a heterogeneous catalyst comprising aluminum orthophosphate.
In the structure (A), the inorganic phosphoric acid includes orthophosphoric acid, metaphosphoric acid and condensed phosphoric acid such as pyrophosphoric acid, and in respect of performance, orthophosphoric acid is preferable. In the structure (B), the organic phosphoric acid represented by formula (1) or (2) includes phosphonic acid, monophosphonate, phosphinic acid, monophosphate, diphosphate, monophosphite and diphosphite or a mixture thereof, preferably phosphonic acid.
The organic group xe2x80x94R in the organic phosphoric acid is preferably an alkyl group such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, n-hexyl, 2-ethylhexyl, octyl, dodecyl and octadecyl, and an aryl group such as phenyl and 3-methylphenyl, to which an amino group, alkoxy group, carbonyl group, alkoxycarbonyl group, carboxylic acid group, halogen group such as chloro group, phosphonic acid group, sulfonic acid group may be added.
In respect of performance and/or cost, the metal atom (C) is preferably aluminum. For the purpose of improving performance such as selectivity etc., the catalyst may contain a small amount of metal atoms other than aluminum, gallium and iron. It is not always necessary that all metal atoms (C) contained in the catalyst are bonded to the structure (A) or (B), and therefore, a part of metal atoms (C) may be present in the form of metal oxide, metal hydroxide etc.
Another preferable example of the weakly acidic solid catalyst of the present invention is a molded, heterogeneous catalyst containing aluminum orthophosphate, preferably having a pore diameter of 6 to 100 nm, a pore capacity of at least 0.46 ml/g, and an acid content of at least 0.40 mmol/g.
The process for producing the weakly acidic solid catalyst in the present invention includes a precipitation method, a method of impregnating a metal oxide or hydroxide with an inorganic phosphoric acid and an organic phosphoric acid, and a method of replacing an inorganic phosphoric acid group of an inorganic aluminum phosphate gel in part by an organic phosphoric acid group. The precipitation method is preferably used.
In preparing the catalyst of the present invention, a carrier having a large surface area may coexist to give the catalyst carried thereon. As the carrier, silica, alumina, silica alumina, titania, zirconia, diatomaceous earth, charcoal etc. can be used. When the carrier is used in excess, the content of the active component is lowered and in consequence the activity is lowered, and therefore the proportion of the carrier in the catalyst is preferably not higher than 90% by weight.
For molding the catalyst of the present invention, a molding additive is also preferably used. As the molding additive, zirconium or titanium hydroxide can be used.
It is preferable that an hydroxide of zirconium or titanium is used as the molding additive, optionally having the formula: MO(2xe2x88x92x/2) (OH)x, M standing for Zr or Ti, X standing for a number of 1 to 4, including at least one hydroxy group. A hydrate of the hydroxide may be also used. Typical examples thereof include zirconium hydroxide, titanium hydroxide, zirconium oxyhydrate, titanium oxyhydroxide and hydrates thereof. Zirconium hydroxide is more preferable from the viewpoint of mechanical strength of the mold.
The molding additive is commercially available. Alternatively it can be produced in a usual manner. For example, a water-soluble compound of zirconium or titanium, such as oxychloride, chloride, nitrate, oxynitrate and sulfate, maybe neutralized with an alkali. An alkoxide of zirconium or titanium may be hydrolyzed.
The weakly acidic solid catalyst of the invention can be produced by mixing powder of a metal salt of a phosphorus-containing acid with a molding additive to obtain a mold article of the metal salt of the phosphorus-containing acid. It is preferable from the viewpoint of improvement in molding and performance of powder to use 2 to 80 wt. %, more preferably 5-50 wt. %, much more preferably 10-50 wt. % of a molding addtive to the dry powder of a metal salt of a phosphorus-containing acid. The amount of the molding additive is determined in term of oxide of zirconium or titanium contained in the hydroxide.
Further a binder of an organic or inorganic compound may be added. Another additive of an organic or inorganic compound or a fine pore-forming agent maybe added to improve plasticity, lubrication and wetting.
The binder may include an inorganic compound such as alminasol, titaniasol, zirconiasol, silicasol and sol of an oxide complex thereof and an organic compound of a cellulose derivative such as methylcellulose and carboxymethylcellulose.
The other additive may include sugars, crystalline celluloses, starches, a polyhydric alcohol such as ethylene glycol, glycerin, sorbitol, polyethylene glycol and polyvinylalcohol, graphite, activated carbon, an organic polymer such as polyethylene, methyl polymethacrylate and nylon, soap such as magnesium stearate and sodium stearate and a surfactant such as an aliphatic alcohol, an aliphatic acid, an aliphatic acid ester, an aliphatic acid amide and polyoxyethylenealkylether.
The mixing can be effected by a usual mixer, for example together with water or an organic solvent such as ethanol, methanol, acetone and 2 -propanol. The temperature and the time is not specified.
The obtained mixed powder product may be molded by a usual molder such as an extruder, a roller molder and a presser. The powder can be improved in fluidity when flowing in the molding machine is an important factor, for example in an extruder or a roller molder. The molded article is not specified in shape.
The molded catalyst produced in the method of the present invention is used usually via a drying step (degreasing step and calcination step). The composition (excluding molding additives if any other than zirconium or titanium hydroxide) of the molded product after drying at a temperature of 200xc2x0 C. or more comprises 20 to 98%, preferably 50 to 95% and more preferably 50 to 90% by weight of a phosphorus-containing acid metal salt. The composition (excluding molding additives if any other than zirconium or titanium hydroxide) of the molded product further comprises 1.2 to 65% by weight of at least one metal selected from zirconium and titanium. In particular when zirconium hydroxide is used as a molding additive, the molded catalyst comprises zirconium in an amount of preferably 1.5 to 59.2% by weight, more preferably 3.7 to 37% by weight and most preferably 7.4 to 37% by weight. When titanium hydroxide is used as a molding additive, the molded catalyst comprises titanium in an amount of preferably 1.2 to 48% by weight, more preferably 3 to 30% by weight and most preferably 6 to 30% by weight.
Further, the composition of the molded product after drying at a temperature of 200xc2x0 C. or more comprises preferably 1 to 80%, more preferably 2 to 50% and most preferably 2 to 30% by weight of molding additives such as binder other than zirconium or titanium hydroxide.
Heretofore, solid acid catalysts have been used in esterification reaction in a fixed bed, and the solid acid catalysts used are those having carriers such as porous materials carrying, or impregnated with, an acid such as sulfuric acid, p-toluenesulfonic acid (PTS), chlorosulfonic acid or methylsulfonic acid, or cation-exchange resin, H-form zeolite, H-form montmorilonite etc. However, these solid acid catalysts are so strongly acidic catalysts that side reactions such as formation of ether derivatives by dehydro-condensation of lower alcohols proceeds. Further, if the reaction temperature is raised-to improve the reactivity, unfavorable side reactions occur further significantly.
The weakly acidic solid catalyst used in the present invention is mainly a catalyst having a weak acid point so that even if the reaction temperature is raised, the progress of side reactions is extremely low, and therefore the reaction temperature can be set high, the temperature of the main reaction can also be raised, and thus the volume of the reactor can be reduced.